1. Field of the Invention
The invention involves articles which depend for their use on photochromic materials.
2. Description of the Prior Art
Articles employing compounds which are photochromic are well known. They are used in articles which change absorption on being exposed to light. Particular examples of commercial interest are glass windows, automobile windshields and eyeglasses. This class of articles has high optical transparency when exposed to only moderate or low intensities of light and then absorb more and more light as light intensity is increased. Optical limiters are another class of articles which depend on this phenomena. The photochromism of certain aziridines are described in H. W. Heine-R. H. Weese-A. Cooper U.S. Pat. No. 3,609,165, issued Sept. 28, 1971.
In the usual photochromic effect, incident radiation (often called excitation radiation) causes the lower-energy form of the photochromic molecule to transform into a higher-energy form of the molecule which has a different absorption spectrum from the lower-energy form. The change in color is due to the fact that the higher-energy form has a different aborption spectrum from the lower-energy form. Thus, excitation radiation can be used to control the absorption of radiation (usually called signal radiation) as for example, in a light switching device operated by the excitation radiation. It should be noted that the excitation and signal radiation may be outside the visible energy spectrum as, for example, in the U.V. or I.R. region.
A characteristic of articles employing photochromic compounds in commercial use is that they respond equally to all polarization of light. Thus these devices cannot be used where it is desired to discriminate between different polarizations of light or to selectively block out light of one particular plane of polarization. For example, in some articles it is desirable to absorb reflected (or glare) light which is predominantly one plane of polarization without significantly affecting the transmission of light with the other plane of polarization. This may be desirable, for example, in sunglasses, in windows, in automobile windshields, etc.
Photochromic articles may also be made into light switches operated by a light source. However, since the light switch is equally responsive to all polarizations of light and the signal absorption does not depend on polarization, the excitation radiation cannot be monitored without affecting the light switch and the signal channel monitored without being absorbed.